There are many factors to consider when fitting a saddle but the two fundamentals are:

1. the comfort and stability of the rider and

2. the comfort and free movement of the horse

- perhaps not in that order!

fitting the rider

The issues around fit of the saddle to the rider depend to some extent on the style of saddle (eg. English, Western and Stock) however the saddle length is always important. It must give the rider enough room for comfort without being so large that the rider slides around on the seat. A good rule of thumb with English style saddles is that there should be about a hand's width of space between the back of the rider's behind and the end of the cantle, as well as a hand's width in the front. Western and Stock saddles tend to have a much tighter fit, about a finger's width. And with stock saddles, there should also be about a finger's width between the rider's thighs and the kneepads (make sure the stirrups are adjusted to the correct length before checking this).

Note that the way in which the length of a saddle is measured varies according to saddle type.  All are measured from the midpoint of the cantle but Western saddles are measured to the back of the pommel,  English saddles  to the nail head (located down from the pommel), and stock saddles to the fore part of the pommel (ie. the raised portion between pommel and facing).  This makes a difference of about one inch between saddle types.

So the same rider might fit a 15" Western saddle, a 16" English saddle and a 17" stock saddle.

(For further info, see Saddle Measurement.)

When the saddle has flaps (English saddles and many Stock saddles), the flap length is another important factor. The flaps should be long enough to be comfortable but not so long that they inhibit the rider's leg contact on  the horse. For example, with a dressage saddle, about two thirds of the rider's lower leg should be in contact with the horse. Saddles with fenders rather than flaps need to have fenders that are of a length that allows them to be adjusted correctly to the rider's leg length.

fitting the horse

Here are some critical issues regarding the fit of the saddle on the horse:

    it should be clear of the spine
    it should be behind the scapula
    it should not rest on the loins
    it should not pinch the sides of the wither
    it should be well balanced with the weight spread evenly.

It's a good idea to check these items both with and without a rider on the horse. 

A horse that's wearing a well fitting saddle experiences less pain, moves more freely and generally performs better for his rider. Often, when a horse is blamed for misbehaviour or poor performance, the problem is actually a saddle that's causing the horse pain or restricting his movement. This can make riding the horse less fun, less rewarding and potentially more dangerous.  A badly fitting saddle can cause severe muscle atrophy (wasting away) which can take a lot of time to correct.  Damage to the nerves caused by a badly fitting saddle may mean that the horse continues to experience pain even with a new correctly fitting saddle.

Some important factors to consider include the width of the saddle, the distribution of weight on the horse's back muscles and the position of the weight-bearing panels relative to the horse's scapular (shoulder blade) and loins.

Another important issue is that the shape of a horse's back doesn't remain constant. So a saddle that fits perfectly today may not fit so well six months from now. For example a horse that's started on a new exercise regime will generally lose fat and bulk up in muscle, changing his shape. Horses lose and gain weight in the different seasons of the year. And older horses can sometimes put on weight (just like their owners!) and they can sometimes lose it.

Whereas the saddle length is determined primarily by the size of the rider, the saddle width relates primarily to the shape of the horse. Horses with a narrow build need a saddle with a narrow gullet while horses with a wide build need a wider gullet. The chamber height and width should be sufficient to  be clear of the horse's spine for the full length of the saddle. (Note that some saddles - including many Western saddles - don't have traditional gullets or panels and tend to fit a broader range of horse shapes however it still must be clear of the spine.

Important: Some experts say that 75% of saddle fit problems are the result of saddles that are too narrow for the horse. A narrow saddle generally sits too high at the front and too low at the rear, whereas a too-wide saddle will generally sit low in the front and high at the rear. A narrow saddle will place lateral pressure on the sensitive area below the wither. It will also tend to push the rider back in the saddle, causing the rider to become unbalanced and create pressure points on the horses back under the cantle.  A narrow gullet channel will also pinch the muscles on either side of the spine, make his back stiff and make his paces shorter and less regular. A saddle that is too wide will sit low at the pommel, placing pressure directly onto the wither and tilting the rider forward.  This creates a pressure point on either side of the wither and also at the front  end of the saddle tree where it rests on the rib shelf.

A good way of assessing width is to take a profile of the horse's back and compare this to the inner shape of the saddle (see box below for information on how to do this).

To relieve the pressure points created by a saddle that's too narrow, some riders try using thicker saddle cloths. This is like putting on a thicker pair of socks when your boots are already too tight - it just makes the problem worse. Saddles that are too wide can sometimes be corrected by use of thicker padding but not saddles that are too narrow.

For a narrow horse with muscle atrophy  beside the wither extra flocking may be needed to lift the gullet free of the wither.  In more modern saddles such as the Bates and Wintec CAIR saddles, flocking is usually not necessary as any adjustments can be made by a Qualified Saddle Fitter using CAIR Panel Inserts above the CAIR bags. 

A rule of thumb for saddles with gullets is that there should be at least three finger's width between the withers and the underside of the pommel.  However this is not hard and fast.  With some saddles, for example Close Contact saddles, much less clearance may be acceptable as long as the gullet and chamber is clear of the spine.

The saddle should also sit level on the horse's back. With an English saddle, this means that the top of the pommel is usually about an inch lower than the cantle, but this varies according to saddle design. A good way of checking whether the saddle is level is to look at the stirrup leathers - they should be hanging straight, not forward or backwards on the saddle. Note too that a jumping saddle often needs to be raised at the back to balance the rider's position in the saddle.

A new saddle whether it is flocked or fitted with CAIR panels will settle.  You should have the saddle re-fitted, and adjusted if necessary, within the first 6 months of use.
 

disclaimer
By providing this information, no advisory, fiduciary or professional relationship is implied or established between the reader and Valley Country. Readers should not treat the information provided here as a substitute for professional advice and should use their own independent judgement.

If you would like to familiarise yourself with saddle fitting issues in more detail, there are some excellent books available that can help.  The Essential Book of Horse Tack & Equipment by Susan McBane has a very useful section on saddle fitting.  Other books on the subject include Horse's Pain Free Back & Saddle Fitting Book by Joyce Harman, Practical Saddle Fitting by Ken Lyndon-Dykes, and the Allen Photographic Guides series called Saddle Fitting.